Thermal analysis software – calculus of the CE and analysis of cast iron transformation

In the previous article “Types of cast iron”, we have introduced the Fe-C phase diagram useful to determine firstly the influence of carbon in the basic Fe-C alloy.
In such article we had as well underlined how it was possible to distinguish, from the Fe-C phase diagram , for each type of cast iron 3 subdivisions by taking into account the carbon percentage in the chemical composition: eutectic cast iron, hypoeutectic cast iron and hypereutectic cast iron.

  • Eutectic cast iron with a carbon percentage (%C) equal to 4,3;
  • Hypoeutectic cast iron with a carbon percentage (%C) from 2 to 4,3;
  • Hypereutectic cast iron with a carbon percentage (%C) above 4,3%.

When evaluating accurately though the cast iron transformation, the influence of other chemical elements must be as well taken into account as discussed in the article “Influence of the alloying elements in the Fe-C Diagram”.

As stated in the literature, without Silicon, the eutectic is equal to 4,3%.
When increasing the Silicon content, the Carbon content needs to be diminished in order to maintain the same eutectic composition. For convenience, the effect of Carbon and Silicon is studied in combination and it is expressed by the index called “Carbon Equivalent”. Such index indicates how accurate and close is the basic chemical composition of cast iron compared to the eutectic point.
Thus, when the Carbon Equivalent is equal to 4,3%, the alloy of the cast iron is eutectic. Instead, when the CE (Carbon Equivalent) has low values of Carbon and/or Silicon the cast iron is hypoeutectic, vice versa, the cast iron represented is hypereutectic.

The formula commonly used for the calculus of CE is as follows:
CE = %C + %Si/3.

Please note that cast irons having the same value of CE can be obtained with different values of Carbon and Silicon. Cast irons with a constant CE can have different quantities of such elements and consequently they will not have similar casting properties.

When other chemical elements influence particularly the alloy of the cast iron, such elements will be included in the calculus of the Carbon Equivalent in order to evaluate not only the effect of Carbon and Silicon but for examples of elements such as primarily the Phosphorus.

The formula commonly used for this specific calculus of CE is as follows:
CE = %C + %Si+%P/3.

With the same method described above, the combined effect of the main chemical elements influencing the transformation of cast iron is calculated. In this case it is also analyzed how close the value obtained is to the eutectic point.

The formula commonly used for this specific calculus of CE is as follows:
CE=%C+ 0,23 (%Si) -0,03 (%Mn) +0,32 (%P) +0,64 (%S) +0,02 (%Ni) +0,06 (%Cr).

Usually, as stated again in the literature, the total contents of the alloy establish the solidification temperature range of the alloy as well as the foundry characteristics of the alloy and its properties.

The CE is even more accurately calculated with instruments such as the thermal analysis software for cast iron (TCAST) directly from the furnace or the pouring line. Such accuracy is guaranteed in the CE calculus with such instrument as specific temperatures acquired during time of solidification of cast iron are integrated in the related formula.

In this way, all the possible elements that can have an influence on the specific cast iron alloy are taken into consideration. In such case, the CE is strictly related to the temperature related to the formation of the first solid nucleus inside the liquid cast iron and it takes into account the basic chemical cast iron composition.

In fact, the phenomenon of the solidification of cast iron is as well monitored from the thermal analysis software for cast iron TCAST that together with the chemical analysis is as well able to forecast accurately the overall metallurgical quality of the cast iron produced.

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